Topic Contents

Electroencephalogram (EEG)

Test Overview

An electroencephalogram (EEG) is a test that
measures and records the electrical activity of your brain. Special sensors
called electrodes are attached to your head. They're hooked by
wires to a computer. The computer records your brain's electrical activity on
the screen. Or it may record the activity on paper as wavy lines. Changes from the normal pattern of electrical activity can show certain conditions, such as seizures.

Why It Is Done

An
EEG may be done to:

Check for
epilepsy and see what type of seizures are occurring.
EEG is the most useful and important test for checking if someone has
epilepsy.

Watch brain activity while a person is getting general
anesthesia for brain surgery.

Help find out if a person has a physical problem or a mental health problem. Physical problems include problems in the
brain, spinal cord, or nervous system.

How To Prepare

Before the day of the
EEG test, tell your doctor if you are taking any
medicines. Your doctor may ask you to stop taking certain medicines before the test. They include
sedatives and tranquilizers, muscle relaxants,
sleeping aids, and medicines used to treat seizures. These
medicines can affect your brain's usual electrical activity. Taking them may affect your test results.

Do not eat or drink things that have caffeine for 12 hours before the test. This includes coffee, tea, cola, and chocolate.

The electrodes will be attached to your scalp. Make sure that
your hair is clean before the test. Don't put sprays, oils, creams, or lotions in your hair. Shampoo your
hair and rinse with clear water the night before or the morning of the test.
Do not put any hair conditioner or oil in your hair after you wash it.

To
find certain types of abnormal electrical activity in the brain, you may have
to be asleep during the test. You may be asked not to sleep at all the
night before the test. Or you may need to sleep less (about 4 or 5 hours) by going to bed
late and getting up early. If your child is going to be tested,
try to keep him or her from taking naps just before the test. If you know that
you are going to have an EEG with little or no sleep, plan to have someone drive you to
and from the test.

How It Is Done

An EEG may be
done in a hospital or in a doctor's office. An EEG technologist does the test. The EEG
record is read by a doctor who is trained to diagnose and treat
problems that affect the nervous system (neurologist).

You will be asked to lie on
your back on a bed or table. Or you may sit in a chair with your eyes closed. The EEG
technologist will attach several flat metal discs (electrodes) to different
places on your head. A sticky paste is used to hold them in place. Instead of separate electrodes, you may wear a
cap with several fixed electrodes. In rare cases, the electrodes may be attached to the scalp with
tiny needles.

The electrodes are hooked by wires to a computer
that records the electrical activity in the brain. A machine can show the
activity as a series of wavy lines on a piece of
paper. Or the activity may be shown as an image on the computer screen.

You will need to lie still with your
eyes closed during the recording. The technologist will watch you directly or through a window
during the test. The recording may be stopped from time to time. This allows you to
stretch and change your position.

The technologist may ask you to
do different things during the test to see what activity your brain does at
that time.

You may be asked to take deep and rapid breaths (hyperventilate).
Usually you will take 20 breaths a minute for 3 minutes.

You may be asked to look at a bright, flashing light called a
strobe.

You may be asked to go to sleep. If you can't fall asleep, you
may get a sedative to help you sleep. If an EEG is being done to
check a sleep problem, your brain's electrical
activity may be recorded all night.

An EEG takes 1 to 2 hours. After the test, you may do your
normal activities. But if you had little or no sleep or were given a sleep medicine,
have someone drive you home after the test.

How It Feels

There is no pain during an
EEG.

Paste may be used to hold the
electrodes in place. Some of the paste may stick in your hair after the test. You will have
to wash your hair to get it out. If needle electrodes are used (which is rare),
you will feel a brief, sharp prick when
each electrode is put in. It will feel kind of like having a hair pulled out. If electrodes are placed in your nose, they may
tickle. Rarely, this may cause some soreness or a small amount of
bleeding for 1 to 2 days after the test.

If you are asked to
breathe fast, you may feel lightheaded or have some numbness in your
fingers. This is normal. It will go away a few minutes after you start
breathing normally again.

Risks

An EEG is a very safe
test. The electrical activity of your brain is recorded. But no electrical current is put into your body. An EEG is not the same as
electroshock (electroconvulsive) therapy.

If you have a seizure
disorder such as epilepsy, the flashing lights may trigger a seizure. Or a seizure may happen if you hyperventilate. If it happens, the technologist is trained to take care
of you during the seizure.

Results

An
electroencephalogram (EEG) is a test that measures and
records the electrical activity of your brain. Special sensors called electrodes are attached to your head. They're hooked by
wires to a computer. EEG test results are ready on the same day or the next
day.

There are several types of brain waves.

Alpha waves are present only when you're awake with your eyes closed but you
are mentally alert. Alpha waves go away when your eyes are open or you are
concentrating.

Beta waves are normally found when you are alert or have taken high doses of certain
medicines, such as
benzodiazepines.

Delta waves are normally found only in young children and in people who are asleep.

Theta waves are normally found only in young children and in people who are asleep.

Electroencephalogram (EEG)

Normal:

In adults who are awake, the
EEG shows mostly alpha waves and beta waves.

The two sides of the brain
show similar patterns of electrical activity.

There are no abnormal bursts
of electrical activity and no slow brain waves on the EEG tracing.

If flashing lights are used during the test, one area of the brain (the occipital
region) may have a brief response after each flash of light. But the brain
waves are normal.

Abnormal:

The two sides of the brain
show different patterns of electrical activity. This may mean that there's a problem in one
area or side of the brain.

The EEG shows sudden bursts of
electrical activity called spikes. Or the test shows sudden slowing of brain waves in the brain.
These changes may be caused by a brain tumor, infection, injury,
stroke, or
epilepsy. When a person has epilepsy, the location and
exact pattern of the abnormal brain waves may help show the type of epilepsy
or seizures. In many people with epilepsy, the
EEG may appear normal between seizures. An EEG by itself does not
diagnose or rule out epilepsy or a seizure problem.

The EEG records changes in the
brain waves that may not be in just one area of the brain. A problem that affects
the whole brain may cause these kinds of
changes. This includes drug intoxication, infections (encephalitis), and metabolic disorders (such as
diabetic ketoacidosis). These problems change the chemical
balance in the body, including the brain.

The EEG shows delta waves or
too many theta waves in adults who are awake. This may mean that there is a brain
injury or brain illness. Some medicines can also cause
this.

The EEG shows no electrical
activity in the brain. This is a "flat" or "straight-line" EEG. This means that brain
function has stopped. It's usually caused by lack of oxygen or blood flow
inside the brain. It may happen when a person has been in a coma. In some
cases, severe sedation from drugs can cause a flat EEG.

What Affects the Test

You may not be able to have the test, or the results may not be helpful, if:

You move too much.

You take certain medicines. This includes medicines used to treat seizures
(antiepileptic medicines), sedatives, tranquilizers, and
barbiturates.

You drank coffee, soda, or tea, or you ate other foods that have caffeine before the test.

You are unconscious from severe drug poisoning or a very low body
temperature (hypothermia).

Your hair is dirty, oily, or covered with hair spray or
other hair products. This can cause a problem with how the
electrodes are placed.

What To Think About

If the doctor thinks that a person has epilepsy but the EEG is
normal, the EEG technologist may have the person look at a
flashing light. Or the person may be asked to breathe fast and deep
(hyperventilate) or sleep during the test. These techniques sometimes show
epileptic EEG patterns that did not show up at first. If epilepsy is suspected
after the first EEG, the doctor may repeat the EEG more than once.

An EEG done during a seizure will almost always show
electrical patterns that aren't normal. This makes an EEG useful when a doctor thinks that a
person is having psychogenic nonepileptic seizures. These are also called pseudoseizures. They have no physical
cause. But they can be caused by stress, emotional trauma, or mental illness.
These seizures do not cause abnormal electrical activity in the brain. They
will not show abnormal EEG results.

There could still be a problem in the brain even when the EEG is normal.

Other tests that may also be done include:

Video EEG. Video EEG records seizures on video and on a computer. This way the doctor can see what happens just before, during, and right after a
seizure. This test can help find the specific area of the
brain that the seizures may be coming from. It also helps in diagnosing
psychogenic seizures, which may look like real seizures but do not affect the
electrical activity in the brain. Video EEG may be used short-term or
long-term.

Short-term monitoring is done on an outpatient basis. It
may last up to 6 hours.

Long-term monitoring is done in the hospital. It may last
3 to 7 days.

Brain mapping. This is a fairly new method that is very similar to
EEG. Electrodes are placed on the person's scalp to transmit the brain's
electrical activity. Then a computer makes a color-coded map of signals from the
brain. It is sometimes done to find a specific problem area in the brain that
has already shown up on a regular EEG. Doctors are still not sure how brain
mapping could be best used.

Ambulatory EEG monitoring. During this test, the person is able to move
around. The test allows for electrical
activity in the brain to be recorded for a long time. Fewer electrodes are attached to the person. The
person carries a small, portable recording unit. The test may last for a
full day or more. The person can leave the hospital. Ambulatory
EEG monitoring is not as accurate as a regular EEG.

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